Below are three possible sources of inspiration for the Zodiac killer. At left, the "Mr. Cipher" villain is from a 1968 comic. The death wheel in the middle (see Zodiac's Halloween card to Paul Avery) "by fire, by knife, by gun, by rope") is from 1954. And at right, the "death machine" book is also from 1968 -- see Zodiac's bus bomb letter.